Process and composition for diffusion coating refractory metals and product produced thereby



United States Patent 3,184,292 PROCESS AND COMPOSITION FOR DIFFUSHON COATING REFRACTORY METALS AND PROD- UCT PRODUCED THEREBY Dimitri Argyriades, Secane, Pan, and Giles 1F. Carter, Wilmington, Del., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del, a corporation of Delaware No Drawing. Filed July 8, 1964, Ser. No. 381,212 20 Claims. (Cl. 29-197) This invention relates to the formation of oxidation resistant diffusion alloy coatings on refractory metals. More particularly, this invention relates to the diffusion coating of refractory metals with certain diffusing elements through a liquid-to-solid transfer process using Group IIA metals as transfer agents.

This application is a continuation-in-part of our copending application Serial No. 57,387, filed September 21, 1960, now abandoned.

Alloys based on the refractory metals, e.g., nickel, cobalt, niobium, molybdenum, tungsten, tantalum and, to a lesser extent, vanadium are used in a variety of applications where high temperatures are encountered. Specifically, turbine blades and buckets, furnace heating elements, jet and internal combustion engine parts of these metals and alloys thereof are characterized by being resistant to chemical attack and having high melting points with the additional characteristic of retaining their good mechanical properties at elevated temperatures. However, in order to prevent the oxidation of these metals at elevated temperatures, it is necessary to exclude oxygen or to protect the metal surfaces with oxidation resistant coatings.

Chromium is also considered to be a refractory metal although oxidation of this metal at high temperatures is not a serious problem. However, a protective coating on this metal serves to resist nitriding.

A number of methods for coating refractory metals are known. Specifically, it has been proposed to protect refractory metals by electroplating following by annealing, vapor or gaseous plating, annealing in pack treatments, dipping, spraying and painting on coatings that can be subsequently annealed. These prior art methods have serious disadvantages in that the protective coatings formed are not sufficiently adherent or they are non-uniform or excessively porous or thick. Frequently these coatings can only be formed under impractical operating conditions and only a limited number of alloys can be prepared.

Coatings can be prepared in accordance with the present invention that are uniform and exhibit excellent adhesion and virtually no porosity. Furthermore, these coatings can be formed on intricately shaped objects and complete coverage, even in the recesses, can be obtained. The large number of distinct alloy coatings that can be obtained as outlined herein is an added advantage.

It is an object of the present invention to provide diffusion alloy coatings on refractory metal articles that resist oxidation under high temperature surface conditions.

It is another object of the present invention to provide unique oxidation-resistant coatings using a novel liquid-to-solid diffusion process.

The diffusion coatings of the present invention are oxidation resistant alloys formed on refractory metal articles by diffusing into the articles one or more diffusing elements from either or both of the following groups: (1) silicon and aluminum and (2) the transition metals having at least one but less than d electrons. Preferred oxidation presistant coatings contain aluminum with or without additional diffusing elements selected from the above-mentioned transition elements.

Aluminum and silicon have the property of being solu- 3,184,292 Patented May 18, 1965 ble to at least some extent in both the liquid transfer medium and the solid refractory article being coated under the conditions outlined herein. In addition, aluminum and silicon impart superior oxidation resistance to refractory metals and can be codiffused with the transition diffusing elements to form alloy rich coatings.

Diffusion of an element is considered to proceed when the concentration of the element in the liquid transfer agent is sufficiently high to favor the elements diffusion into the refractory article. Considering the mechanism as principally depending on the relative chemical activities of a particular element in both the transfer agent and the solid article, it is observed that a high concentration or a saturated solution of a diffusing element in a liquid melt of the transfer agent favors diffusion of the element into the refractory article when the element is soluble but not present in saturated amounts in the solid article.

Diffusion can be carried out by contacting, preferably by immersion, the refractory metal with a melt of the transfer agent containing one or more of the diffusing elements at a temperature of about 900-1450 C., preferably at about 1000l400 C.

The preferred transfer agent is calcium although other Group HA metals such as barium and strontium, and mixtures thereof, are suitable. The high vapor pressure of magnesium at operating temperautres makes it less desirable as a transfer agent. These metals, except for beryllium, have melting points below 900 C. and are comparatively inert toward all of the refractory metals except nickel. Where nickel-rich alloys are used as substrates, it is necessary to maintain suflicient nickel dissolved in the transfer agent to prevent dissolution of the substrate. The diffusing elements are all soluble to the extent that diffusion is favored when sufiicient concentrations of the diffusing elements are incorporated in the IIA metal transfer agents.

At least about 25% by weight of the diffusion composition (transfer agent and diffusing elements) should be comprised of the transfer agent; up to 99.5% of the composition can be the transfer agent, but the preferred amount of the agent is about 35-65% by weight. When aluminum is being diffused from a Ca-Al composition, preferably about 35-65% of the diffusing composition should be calcium, the balance being aluminum or aluminum and up to about 30% additional diffusing elements. In general, only small amounts, about 0.5% by weight, of the diffusing elements need be present in the composition and up to about can be used.

The second group of diffusing elements include those transition elements that have at least 1 but less than 10 d electrons. Specifically, titanium, manganese, iron, yttrium and zirconium are embraced within this group. In addition, this group of diffusing transition elements includes the refractory metals vanadium, chromium, cobalt, nickel, molybdenum, niobium, tantalum and tungsten which are preferably co-difiused with at least one other diffusing element, usually aluminum, to form oxidation resistant coatings on refractory metal articles.

It should be emphasized that the majority of the diffusing elements are sparingly soluble in melts of the transfer agents, the most notable exceptions being aluminum, silicon, and nickel. Introducing these slightly soluble diffusing elements as finely divided powders (preferably -400 mesh) and providing agitation during dif fusion serve to facilitate the formation of the coating. It should also be noted that during diffusion, multiple component systems exist in both the coated article and in the diffusion composition. These systems have characteristic melting points and solubilities that are distinct from those of the starting materials.

Frequency, these distinct melting points and solubility changes can be advantageous. For instance, some coatin'gs are'formed to an appreciable extent only in thepresence of certain other. metals. Chromiumfrich coatings can be formed on niobium when other diffusing elements such as Al, Ni or Co are present in the diffusion mixture; whencalcium-chromium' dilfusion mixtures are usedwithout additional metals,-rnuch, but'notgall, of thedeposite'd Cr adheres'jwithout' tdiffusing into the refractory metal'substrate. "Other elements such as yttrium; vanadium, molybdenum, titanium, and zirconium will form diifusion'alloy coatings with aluminum on niobium in Ca-Al baths. serves to increase the solubility of theserc'omparativcly insoluble elements in calcium or in the substrate; This is not to; say, however, that these comparatively insoluble elements; ca'nnotbe diffused to someextent'into' niobiumand other refractory metal articles frorn: a melt of calcium inthe absence ofaluminum. Rather, it) is tostate that these elements should be codiffused with aluminum in jorder to obtain vcoatings sufliciently alloy rich to impart appreciable resistance to oxidation at elevated temperatures; moreover, 'all-oy coatings containing aluminum are preferred.

The following examples illustrate specificembodiments of the present, invention. 7

EXAMPLE I Samples of'Moand W were treated without stirring in a bath containing60 g.'Ca, 6 g. Cr, and 10g. Ni in a carbon steel crucible. The samples were heated for about'l hour at l290'C. The coatings .on the Mo and The presence of; aluminum apparently a 'VE XA MPLEV 7 Samples of NbQMo and W'weretreated in a .bath containing 30 g. Ca, 5' g. Cr, 5 g. Co, and 3 g. Y for about 2 hours at1300f C. The samples were not agitated in the bath. The Nbsample. had a' high concentration of Co, a few percent of Cr, andabout 1% ofY at the surface of the"coatin'g.-/The Mo and had a high concentration ofCo, several percent of Cr, and about 16 2.%'3 Y at the surface" of the coating. I

" EXAMPLE v1 Nb was coated with V and Ni. by treating it in a bath containing about145 g;{Ca,, 3 g. Ni, and '5 g. V (20 'mesh). The samplewas heatedfor 3 hours. at about.

1200 Ciinlan induction -fu'rna,ce H g .:EXAMPLE'VII A niobiumarticle was treatedin a melt containing 54 g. of Ca-Al alloy, 73% Ca 27 %{*Al; tow-hichhadbeen added grams" of 'j The melt was contained in a 1 Mo crucible and the samplewas treated'for 4hours at 1300 C. The Nb-Al alloycoating obtained withstood oxidation, for more than 56 hours whcn'heated to 1050 C. in air; V

' p EXAMPLE VI I A'sample of Nb was coated with appreciable concen- I trations of Cr, Mn, C0, and Niby treatment in a bath W samplesyboth contained'nearlythe same concentrationsof Cr (over 50%), Fe (over 10%), and Ni (over 10%). These coatings impart oxidation resistanceto the refractory metal substrates, but the resistance to-oxi- ,dation is not'as. high as those alloy coatings containing aluminum.

Samples of Nb, ring in. abath containing 25 g. Ca, 3 g. Mn, .3 g. Co, and 5 g; Crpowder. in a Mo crucible They were heated at 1290" C.- for about 2' hours- The Nb sample was coated with highgconcentra'tions of Ni, Co, and Mn (each 10% and with "several percent of. Cr.

of Ni, Co,"and Mn and a lower concentration ofQCr.

EXAMPLE 11' 1 v V Mo and W were treated without stir-' The Mo andW samples contained similar high concentrations The Ni was presentas a contaminant in the Mo crucible from a previous run inwhi'ch the M0 crucible was'used to: hold a Ca-Ni bath. t 7

' EXAMPL 111 When Nb, Mo, and .W were treated; without agitation in a bath; containing'75 'g. Ca, 10g; Cu, and 10; g; Zn, and placed. in a mild steel. crucible, at 1290" C. for about 2 hours, the coatings on the samples contained ironzbut3little, if any,*Cu' or 'Zn. .Copper and..zinc,can. be use with? calcium and serve :to dilute thedifiusion compositionsince neitherqcooper nor zincdiifuses into the refractory. metalsubstrateto any extent and these metalszre'm'ain substantially inertin the melt;

V "EXAMPLEIV Sampls of1Nb,;Mo andW were treated without agita-' tion in a bath containing 15 g.- Ca, 15 g. CaSi and 5 "g. Cr powder for 2'h0urs at.,1300 C. The'coatings on the fNb, Mo, and W samples contained bothiCf and Si,

identified byX ray'fluorescence. Alloy content and thick-v nesses of'the various: coatings obtained are determinedv byalternatelyl examining the'. coated article with X-ray.

containing aboutx ,g; Ca and 0.5g. Co, 0.5 g. Mn, 0.5 g. Ni, and 5 'g. Cr atabout,1300 C.,'heated in an induction furnace for 40 minutes.

' EXAMPLE IX 7 Four samples. of niobium were immersed in a, melt comprised of 70 grams ,Ca 50% Si'for ,4 hours at 1300 C} All samples gained appreciable weight and the coatin'gs were 1.7-2.5 mils thick and multilayered' withdistinct alloys. When heated in air to 1050 C the coatings beganto'fail only after 17 hours. Complete failure was -not.;observed, ,butfseveral spots appeared 'indicating that the coating was penetrated. In the oxida- 7 tion test, breakdown'of coating usually occurs at a corner or along the edge. I

EXAMPLE x A' sample. of Nb was coatedvwith Ni by treatment in 45 g.-;Ca+5 g. Ni shot for 4-.hours at-1300 C. The.

sample contained 30% Ni-at-the surface.

EXAMPLE XI" A sample f Nb was coated with Ni and A1 by treatment for 4 hours in 54 g. 73--27%- Ca-Al alloy, 15 g. Al, and 21g. Ni= The temperature was 1300 C. The

sample contained large concentration 20%) each of Ni and A1. a EXAMPLE XII I A sample of Nb wastreated in a Mo container hold- 7 ing a bath composition ,of' 74 g. of 50% Ca'50%' Al alloy' 'andj, g. Ti powder for 4 hours at l300 C. in

V argon. The coating contained Al, Nb, Ti, and M0. The

concentration of Ti was greater, than. 20%. Some of the Mo from the container diffused, intov the Nb sub .strate.

, EmMPLE-XHI V A sample of Nb was treated in. a bath containing 74 g. 50% Ca50% Al alloy and a 2, g. piece of zirconium for 4 hours'at 1300? C. inargon. Thediffusion coating that formed contained mainly Nb 1 and A1 with about sample. was-treatedin a bath containing 74 g.--.50-50 Ca-Al and :3, g. Zrpowder for14: hours at 1300-"; C. in. Thejcoating containeda-high concentration of% argon. 7 Al (20-30%), over 15% Zr, and about 5% M0. The

Fe was present as an impurity. from the apparatus or in the starting material,:probably1the Ca-Al alloy, and the.

source of Mo .was. thecrucible container.

5 EXAMPLE xrv A sample of Nb was treated in a bath containing 74 g. 50% Ca50% Al and 5 g. Fe powder for 4 hours at 1300 C. in argon. A Mo crucible was the container. The coating on the Nb sample contained high concentrations 20%) of Al and Fe, as well as over 10% Mo.

EXAMPLE XV Samples of Nb, V, Ta, and M were treated in a bath containing 79 g. of 50% Ca50% Al alloy in a Mo crucible for 3.5 hours at 1300 C. under argon. A coating of Al-Mo-Fe-Nb was obtained on Nb. The coating on V contained primarily Al, Mo, Nb, and smaller concentrations of Ta, V, and Fe. The coating on Ta contained Al and small concentrations of Mo, Nb, and Fe. The coating on Mo contained Al and small concentrations of Ta, Nb, and Fe. Fe was present in the system as an impurity.

EXAMPLE XVI Samples of V, Ta, and Nb sheet were treated in a bath containing 45 g. Ca, g. Cr powder, 2 g. Ni, 2 g. Co, and 2 g. Mn in a Mo crucible for 4 hours at 1300 C. in argon. The vanadium had a thick coating which contained appreciable concentrations by weight) of Cr, Mn, Co, Ni and smaller concentrations of V and Nb at the surface. The tantalum was coated with an alloy containing Cr, Mn, Co, Ni, and Nb. The Nb sample was coated with appreciable concentrations of Cr, Mn, Co, Ni and a trace of Ta.

EXAMPLE XVII A sample of Nb was treated in a bath containing 32 g. Al, 27.5 g. Ba, and 52.5 g. Sr in a Mo crucible. The sample was heated for 4 hours at 1300 C. in argon. The coating was similar to that in Example XXIII. A small concentration of Sr was detected throughout the coating -by X-ray fluorescence. For analytical purposes the coating was partially removed by electropolishing. Electropolishing was alternated with X-ray fluorescence in determining the depth to which Sr penetrated.

EXAMPLE XVIII A sample of Nb-Mo-Ti alloy (80% Nb10% Mo10% Ti) was treated for 18 hours in 82 g. 50% Ca-50% A1 alloy at 1100 C. in argon. The sample had a coating of 2.5 mils. The structure was nearly the same as that obtained in Example XXIII. However, the layer having the Nb Al structure was about 0.4 mil thick. The surface of the coating was poorer in Ti and richer in M0 than the starting alloy. The surface also contained Al and Nb and a trace of iron. This sample lasted for 42 hours at 1050 C. in air before rapid oxidation began.

EXAMPLE XIX A sample of Nb was treated in a bath containing 82 g. of 50% Ca-50% Al alloy and 0.5 g. Ni as outlined in Example XXI. The coating, after about 18 hours of treatment, was 35:0.2 mils with a Knoop hardness of 725 to 840. The grains are mainly columnar and are extremely narrow. The coatings structure was similar to that obtained in Example XXIII. This sample resisted oxidation for 118 hours at 1050 C. in air.

EXAMPLE XXI A sample of Nb was placed in a bath containing 82 g. of 50% Ca-50% Al alloy and 0.5 g. Co contained in a molybdenum crucible. The sample was treated for about 16 hours at 1100 C. in argon. The niobium was then removed, cleaned, and analyzed by X-ray fluorescence. The surface contained about 10% M0, 0.5% Fe, l-2% Co, 20-30% Al, and the balance niobium. The coating was 2.3 10.2 mils thick. Its structure was similar to that obtained in Example XXIII. The sample resisted oxidation at 1050 C. in air for hours without any breakdown of the coating.

EXAMPLE XXII When a sample of Nb was treated for 16 hours in 66 g. 61% Ca39% Al alloy at 1100 C. in argon, the sample was similar to that in Example XXIII; that is, it had a relatively thick coating having the NbAl structure and a thin layer having the structure of Nb Al.

EXAMPLE XXIII A sample of pure Nb (about 1%" x x 0.04) was rotated in 82 g. 50% Ca-50% Al alloy for 18 hours at 1100 C. in an argon atmosphere. The bath was contained in a niobium crucible. The sample was removed, cooled, and cleaned in dilute HCl. A diffusion alloy coating of aluminum-niobium (about 3 mils thick) was prescut on the niobium substrate. This sample was heated in air at 1050 C. for 117 hours without appreciable oxidation and with no oxygen penetration. The sample gained 56 mg. in 117 hours due to oxidation, or less than 0.1 mg./cm. per hour.

Another sample of niobium, which had a similar treatment in 50% Ca-50% Al alloy, was cross-sectioned, polished, etched, and photographed. Two layers were revealed: (1) the upper layer, about 2.7 mils thick, had the crystal structure of NbA13, determined by using an X-ray difiractorneter and (2) a thin layer, 0.2-0.3 mil, appeared between the top layer and the niobium substrate. This layer has the structure of NbgAl.

EXAMPLE XXIV A melt of a diffusion composition containing 81 g. Ca, 3 g. Mn, 3 g. Fe, 3 g. Co, and 10 g. Ni was heated to about 1100 C. A sample of Cr was rotated for 2.5 hours in the bath, contained in a carbon steel liner. The Cr was coated with Fe, Ni, Co, and Mn and the coating was about 0.002" thick.

The processes described in the above examples were carried out in an inert atmosphere of argon. Although diffusion can be carried out without the protection afforded by an inert atmosphere, it is preferred to exclude oxygen, nitrogen, and other contaminants, particularly at temperatures higher than 1100 C. and when the diffusion composition is comprised of aluminum and calcium. A practical method of carrying out the process at high temperatures is to enclose the melt within a container. The air can be exhausted from the container or simply permitted to become dead without replenishment during operation. It is to be understood, however, that diffusion can be carried out in the absence of the above measures and one skilled in the art can readily determine the most feasible conditions under which a particular refractory metal substrate can be coated with a particular alloy coating. In the event an inert atmosphere is desired, other gasses such as helium will suggest themselves. After the treated articles are removed, it is preferred to dissolve away the melt residue with dilute I-ICl or other suitable reagent.

EXAMPLE XXV A series of experiments was conducted in which base refractory metals having the compositions indicated in Table 1 which follows were immersed in a stirred molten metal bath under an inert argon atmosphere. The bath consisted of calcium and one or more diffusing elements as indicated in the table. Upon removal of the coated article from the bath, quenching was accomplished by 7 either exposure to air at room temperature or immersing the article in an oil bath. Each of the coatcd'articles was cleaned and then analyzed to determine its composition. The final percentage of diffusing elements inthe surface ofthe coated article is recited in the table. .The determination of the surface composition in this and the foregoing examples.wasaccomplishediin the usual manner 8" coating. Thus the oxidation resistance conferred by these superior phase structures can be supplemented by further benefits derived from specific 'codiifusing elements." Furthermore, these desirable phase structures can be maintained on niobium base alloys despite the outward diffusion of alloying elementspresent in the basis metal. 1 Concentration gradient analysis has shown that these codiifusing elements. are likewise present in concentration gradients that decrease toward the interior of the coating.

Apparently the relatively .thin Nb Al layer is a diffu- {sion barrier and theVrate of diffusion is slower through this layer than in the relatively thick layer of NbAl The fact that this thin layer, is .a diffusion barrier accountsin partfor the superior oxidation resistance of the Nb-Al coatings. This is evident becauseqwhen heated in air at IO50 C. the aluminum in the coatings diffuses Table 1 t Diffusing Base Metal Composition Molten Bath Composition Coating Percen (percent) (gms,) Operation Elgntenlts 1n (floated V Quench 1' 10 e Sur ace Experiment ing Medium- Ni Cr Mo W Fe Ca Ni 00 1 Crl Al Ni too Cr Al 1 -325 mesh powder.

It is to be noted that a great number of the coatings formed are novel in terms of alloy content and, in general, it can be said that the oxidation resistance of the refractory metals is enhanced by the great majority of the diffusion coatings obtained although the degree of resistance of individual/coatings vary considerably. Isolated instances where low melting alloys are formed or where theoxidation resistance .of the coated refractory metal is not significantly increased can probably be cited. By following the examplesyhowever, one, skilled in the art can determine the conditions under which suitable coatings can-be obtained.

The best coatings obtained in terms of oxidation resistance were those containing aluminum and, to a less extent, silicon. From the standpoint of refractory materials treated, undoubtedly those .articles formed of niobium, molybdenum and tungsten or alloys, of these metals have the greatest utility and will be more widely used than other refractory metals. In particular, coatings containing aluminum on niobium; or alloys of niobium are outstanding in terms ofoxidation resistance, .and the base metal remainsductile'because oxygen does 'not penetrate through the coatings.

The nature of the superior oxidation resistant coatings obtained in Example XXIII and similar coatings has been extensively investigated. As noted above, these coatings I are comprised of layers having distinct phase structures; the thicker upper layer having a structure corresponding to NbAl and the relatively thin bottom layerhaving a structure corresponding toNb Al. Throughout the en tire coatings and in bothlayerstheialuminum and mic In the bium are present in concentration gradients. -NbAl -phase layer the concentration of aluminum decreases slowly with increasingdepth until the thin inner layer is reached.v Thereafter the concentration of alumi- -diifusing elements, including W, Fe, Mo, Co, iNi and others, do not change the structures of the phases of the very slowly, even more slowly than the rate of coating formation. In other words, the coating remains alloy rich and relatively fixed in content and structure. Photomicrographs and microhardnesses of these multilayered coatings that, have been subjected to the oxidation "test ihow that, oxygen penetration is arrested in the NbAl ayer.

In addition toalternately scanning the surfaces of the coatings with a ditfractometer and polishing to remove that area examined, a careful examination of photomicrographs has established that these two layers having distinct phase structures exist. The photomicrographs show that many of the grainsare extremely narrow and columnar. Examination with the diffractometer establishes the structure of the thick outer layer as corresponding to that reported in. the literature for NbA13. As the coating is polished and examined, the change of intensity of X-ray diffraction peaks indicates that a wide range of solid solubility exists in the Nb-Al coating. Also the results of X-ray investigations reveal the shifting of position of peaks as one probes deeper into the coatings. Under similar examination, the thin inner layer has been identified byX-ray refraction as having the same structure as Nb Al which'is also reported in the literature in the form of bulk or substantially homo geneous alloys. I

The multiphase coating is obtained in accordance with the preferred process wherein about 35-65% by weight of the diffusion composition is calcium and about 35- by weight is aluminum. As the concentration of calcium is increased beyond approximately 65% by of the diffused elements, the highestconcentrations being present in the outer portions of the coatings. The coatings also contain small amounts of the transfer agents which are present in a similar gradient. Generally, the amount of transfer agent is very small, less than -1% by weight, but even small amounts of Ca, Ba, and Sr have been reported to augment resistance to oxidation.

In the foregoing description and examples, parts and percentages are by weight unless otherwise indicated.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for diffusion coating a base refractory metal comprising contacting said metal with a molten composition containing at least about 25% by Weight of a member selected from the group consisting of calcium, barium, strontium, and mixtures thereof, as a transfer agent, and from 0.5% up to 75% by Weight of at least one difiusing element selected from the group consisting of aluminum, silicon, and transition elements having 1 to 9 d electrons, said process being carried out at a temperature of about 900-1450 C. for a time sufficient to form a diffusion alloy coating.

2. A process for diffusion coating a molybdenum base refractory metal comprising contacting said metal with a molten composition containing about 35-6S% by weight calcium, about 35-65% by weight aluminum, and up to about 30% by Weight of at least one diffusing transition element having 1 to 9 d electrons, said process being carried out at about 900-1450 C. for a time sufficient to form a diffusion alloy coating.

3. A process for diffusion coating a base refractory metal comprising contacting said metal with a molten composition containing at least about 25% by weight of a member selected from the group consisting of calcium, barium, strontium, and mixtures thereof, as a transfer agent, up to about 75% by weight of at least one diffusing element selected from the group consisting of aluminum and silicon and up to about 75 by weight of at least one diffusing transition element having 1 to 9 d electrons, the total amount of diffusing material exceeding 0.5 by weight, said process being carried out at about 900-l450 C. for a time sufficient to form a diffusion alloy coating.

4. A process for diffusion coating a niobium base refractory metal comprising contacting said metal with a molten composition containing at least about 25% by weight of a member selected from the group consisting of calcium, barium, strontium, and mixtures thereof, as a transfer agent, up to about 75% by weight of at least one diffusing element selected from the group consisting of aluminum and silicon and up to about 75% by weight of at least one diffusing transition element having 1 to 9 d electrons, the total amount of diffusing material exceeding 0.5 by weight, said process being carried out at about 900-1450 C. for a time sufficient to form a diffusion alloy coating.

5. A process for diffusion coating a tungsten base refractory metal comprising contacting said metal with a molten composition containing at least about 25% by weight of a member selected from the group consisting of calcium, barium, strontium, and mixtures thereof, as a transfer agent, up to about 75 by weight of at least one diffusing element selected from the group consisting of aluminum and silicon and up to about 75 by weight of at least one diffusing transition element having 1 to 9 d electrons, the total amount of diffusing material exceeding 0.5 by weight, said process being carried out at about 900-l450 C. for a time sufficient to form a diffusion alloy coating.

6. A process for diffusion coating a molybdenum base refractory metal comprising contacting said metal with a molten composition containing at least about 25% by weight of a member selected from the group consisting of calcium, barium, strontium, and mixtures thereof, as a transfer agent, up to about 75% by weight of at least one diffusing element selected from the group consisting of aluminum and silicon and up to about 75 by weight of at least one diffusing transition element having 1 to 9 d electrons, the total amount of diffusing material exceeding 0.5 by weight, said process being carried out at about 900-1450 C. for a time sufficient to form a diffusion alloy coating.

7. A process for diffusion coating a base refractory metal comprising contacting said metal with a molten composition containing at least about 25% by weight calcium, up to about 75 by weight of at least one diffusing element selected from the group consisting of aluminum and silicon, and up to about 75% by weight of at least one diffusing transition element having 1 to 9 d electrons, the total amount of diffusion material exceeding 0.5 by weight, said process being carried out at about 1000-1400 C. for a time sufficient to form a diffusion alloy coating.

8. A process for diffusion coating a base refractory metal comprising contacting said metal with a molten composition containing at least about 25% by weight calcium, up to about 75 by weight of at least one diffusing element selected from the group consisting of aluminum and silicon, and up to about 75% by weight of at least one diffusing transition element having 1 to 9 d electrons, the total amount of diffusion material exceeding 0.5 by weight, said process being carried out at about 900-1450 C. for a time sufiicient to form a diffusion alloy coating.

9. A process for diffusion coating a niobium base refractory metal comprising contacting said metal with a molten composition containing at least about 25% by weight calcium, up to about 75 by weight of at least one diffusing element selected from the group consisting of aluminum and silicon, and up to about 75 by weight of at least one diffusing transition element having 1 to 9 d electrons, the total amount of diffusion material exceeding 0.5 by weight, :said process being carried out at about 1000-l400 C. for a time sufficient to form a diffusion alloy coating.

10. A process for diffusion coating a base refractory metal comprising contacting said metal with a molten composition containing at least about 25 by weight calcium, up to about 75% by weight of aluminum, and up to about 75 by weight of at least one diffusing transition element having 1 to 9 d electrons, the total amount of diffusion material exceeding 0.5% by weight, said process being carried out at about 1000-l400 C. for a time sufficient to form a diffusion alloy coating.

11. A process for diffusion coating a base refractory metal comprising contacting said metal with a molten composition containing about 35-65% by weight calcium, about 35-65% by weight of at least one diffusing element selected from the group consisting of aluminum and silicon, and up to about 30% by Weight of at least one diffusing transition element having 1 to 9 d electrons, said process being carried out at about 1000-1400 C. for a time sufficient to form a diffusion alloy coating.

12. A process for diffusion coating a base refractory metal comprising contacting said metal with a molten composition containing about 35-6S% by weight calcium, about 35-65% by weight aluminum, and up to about 30% by Weight of at least one diffusing transition element having 1 to 9 d electrons, said process being carried out at about 1000-1400 C. for a time sufficient to form a diffusion alloy coating.

13. A process for diffusion coating a niobium base refract-ory metal comprising contacting said metal with a molten composition containing about 35-65% by weight calcium, about 35-65% by weight aluminum, and up to about 30% by weight of at least one diffusing transition element having 1 to *9 d electrons, said process being carried out at about 900-1450 C. for a time sufficient to form a diffusion alloy coating.

14. A process for diffusion coating a tungsten base reof at least one diffusingtransition elementhaving 1 to 9 d electrons.

1 6. A refractory metal article-having an oxidation resistant eoating comprising a refractory metal alloycontaining at least one diffused element selected from the group consisting of aluminum, silicon, and transition ele ments having 1 to 9 d electrons, said coating'having increasing concentrations of said diffused elements and a transfer agent toward the outer portion thereof, said trans- .feragent being selected from-the group consistingof calcium, strontium, and barium.

17. A refractory metal article having an oxidationvresistant difiusion coating comprising a refractory metal alloy containing aluminum and a transfer agent selected from the group consisting of calcium, strontium, and barium, said coating having increasing concentrations of said aluminum and said transfer agenttoward the outer portions thereof.

18. A refractory metal article having an-oxidatiion resistant coating comprising a refractory metal alloy containing at least one diffused element selected. from' the group consisting of aluminum and silicon, said coating having increasing concentrations of saidditfused material and a transfer agent toward the outer portion thereof,.said

transfer agent being selected from the group consisting of calcium, strontium, and barium.

19. A niobium base refractory metal article having an oxidation resistantcoating comprising a niobium refractory metal alloy containing at least one diffused element selected from the groupconsisting of aluminum and silicon, saidcoating having increasing concentrationsof said diffused material and a transfer agent toward the outer portionthereof, said transfer agent being selected, from the groupconsisting of calcium, strontium, and barium.

20. A niobium base refractory metal article having an oxidation resistant diffusion coatingcomprising a niobium refractory metal alloy containing valuminumand a transfer agent selected from. the group consisting of calcium, strontium, and barium, said coating'havingincreasing concentrations of said aluminum and said transfer agent towardv the .outer portions thereof.

References Cited by the Examiner UNITED STATES PATENTS 1,683,086 9/28 Meehan -138 X 2,223,977 12/40 I Wamsley 250-275 2,257,988 ,10/41 Suchy et al. 75-1-38 2,525,263 10/50 Macksoud ,117-333 2,686,735 8/54 Thomas 11'7-3326 2,771,666 11/56 Campbell '29-198 2,788,289 4/57 Deuble 1'17-65 3,055,088 9/62 Cox .29-194 3,061,462 10/621 Samuel 117-107 3,069,288 12/62 OXX et al. 117-71 3,078,554 2/63 Carlson 29-194 FOREIGN PATENTS 574,092 12/45 GreatBritain.

OTHER ,REFERENCES I I Gittings et al.: Trans. :Amer. Soc. for Metals, vol. 43

v(195-1), 5 6.593 and 59.4. 1

Mantellzand Hardy, Calcium, ACS Series (1945) page '35.

:RICHARD =D. NEVIUS, Primary Examiner, 

16. A REFRACTORY METAL ARTICLE HAVING AN OXIDATION RESISTANT COATING COMPRISING A REFRACTORY METAL ALLOY CONTAINING AT LEAST ONE DIFFUSED ELEMENT SELECTED FROM THE GROUP CONSISTING OF ALUMINUM, SILICON, AND TRANSITION ELEMENTS HAVING 1 TO 9 D ELECTRONS, SAID COATING HAVING INCREASING CONCENTRATIONS OF SAID DIFFUSED ELEMENTS AND A TRANSFER AGENT TOWARD THE OUTER PORTION THEREOF, SAID TRANSFER AGENT BEING SELECTED FROM THE GROUP CONSISTING OF CALCIUM, STRONTIUM, AND BARIUM. 